JP2005-199908A discloses a battery state determination device for an electric power steering device comprising a first actuator driven by a main battery and a second actuator driven by a sub-battery, wherein if a lack of driving power occurs in steering wheels by means of the first actuator, the driving power of the first actuator is assisted by the driving power of the second actuator, characterized in that said battery state determination device is configured to determine an abnormality of the main battery if an amount of driving the second actuator exceeds a predetermined threshold.
However, in a power supply system using two or more than two different batteries, it is necessary to detect not only an abnormality of the main battery as the aforementioned prior art but also the state of the sub-battery. On the other hand, in order to detect the state of the sub-battery, it is necessary to form a power supply condition suited for the detection of the state.